In the design and assembly of d.c. motors and generators that use permanent magnet segments in the stator design, the magnet segments must be held in place in the stator structure for proper motor operation. At present, there are several ways of building motors or generators and keeping the magnet segments in place.
One concept involves only the use of adhesive bonding of the magnet segments to the outer stator housing or yoke. The magnet segments may be held in place only through the use of an adhesive bonding agent, or the adhesive may be used in conjunction with the other concepts discussed below. When the magnet segments are bonded together only with an adhesive bonding agent and the adhesive bond fails, a magnet segment will fall onto the armature preventing the armature from rotating, and causing the electric motor or generator to fail.
Adhesive bond failure can occur through various causes. The most notable causes of adhesive bond failure are improper surface preparation, improper adhesive mixing, excessive voids trapped in the adhesive between the magnet segments and the yoke, corrosion formed on the surface between the yoke and the magnet segments where bonding is to take place, and trapped moisture in the adhesive.
Another concept involves an arched magnet segment construction. This construction is sometimes called a "keystone" construction, and depends upon the wedge shape of the individual magnet segments to hold the assembly together. While this construction does work, it requires that very close tolerances be maintained on all joining pieces to keep the magnet segments from falling onto the armature. The air gap between armature and stator magnet is typically very small, i.e., 0.010 inch (0.25 mm). The smaller the air gap, the closer the tolerance required for the wedge shaped pieces. There is another disadvantage of this design when it is used in conjunction with the use of an adhesive because, when the last wedge is inserted, all or most of the adhesive will be wiped from it because of the tight fit required between the pieces.
A third concept involves a notched magnet segment/wedge construction. This construction reduces the tolerance problem of the design just discussed, but it requires that a step or notch be put into the edge of the magnet segments. The provision of such a step or notch may not be possible on thin magnet segments. Furthermore, the magnet material which is brittle, tends to crack in the location of the notch.
A fourth concept involves a spring clip construction. In this construction, the magnet segments are wedged together, and a spring is used to apply a spring force on the brittle magnet edge surface. This construction is difficult to accomplish successfully with thin magnet segments. Furthermore, it is not an ideal solution in high vibration or high temperature environments, because the spring clip can fatigue and/or degrade the mating surface of the magnet material.
A fifth concept involves a cradle construction. This construction cradles all of the magnet segments in a structure that is then attached to the yoke. This construction is essentially a structure within a structure. This construction is very expensive and may require a larger air gap to clear it. In order to accommodate a larger air gap, a larger magnet must be used, and this results in additional cost and well as space considerations.